A problem of a closed insulating device such as a gas-sealed switchgear in which a high-voltage conductor supported by an insulator is provided in a container having an insulating gas sealed therein, is miniaturization by rationalization of insulation design or three-phase integration for cost reduction and environmental load reduction.
The size of a metal container of the closed insulating device is decided by insulation design, thermal design or the like. One of points of the insulation design is to discuss the influence on the insulation performance in the case where foreign substances exist (adhere) on the inner surface of the metal container.
When foreign substances exist inside the metal container in which the high-voltage conductor supported by the insulator is housed and the insulating gas is sealed, force is generated to the foreign substances due to interaction between electric charges supplied from the metal container or the like and an operation voltage. Therefore, the foreign substances sometimes move around inside the metal container.
When the closed insulating device is miniaturized, the electric field on the inner surface of the metal container becomes high and the movement of the foreign substances existing inside the metal container are thus likely to become active. The foreign substances, if excessively moving inside the metal container, sometimes affect the insulation performance. Further, with a longer shape of the foreign substance, the foreign substance more greatly moves and thus more greatly affects the insulation performance.
For this reason, to prevent the long foreign substance from entering the inside of the metal container, for example, a foreign substance management process is provided to remove the foreign substances in a manufacturing process to thereby enhance the management of the foreign substances. Further, it is necessary to design the electric field intensity on the inner surface of the metal container at the application of the operation voltage to prevent a small foreign substance that is difficult to manage from floating to a height taken into consideration in design or higher and moving around. Here, the height means the distance between the inner surface of the metal container and the foreign substance.
The electric field intensity on the inner surface of the metal container depends on the distance between the high-voltage conductor and the inner surface of the metal container. Therefore, to suppress the floating height of the foreign substance, it is necessary to increase the size of the metal container. This becomes a factor in hindering the miniaturization of the closed insulating device.
There is, as a method of mitigating the influence by the foreign substances, a method of suppressing the movement of the foreign substances by coating the inner surface of the metal container with an insulation resistant material. By coating the inner surface of the metal container with the insulation resistant material, supply of electric charges from the inner surface of the metal container to the foreign substances is suppressed to make the foreign substances hard to move. In this case, the specific volume resistivity of the insulation resistant material for coating needs to be controlled to a specific volume resistivity capable of suppressing the movement of the foreign substances.
However, in the case of using a conventional insulation resistant material, electric field concentration is likely to occur at a portion composed of the foreign substance, the insulating gas, and the insulation resistant material. When the electric field concentration becomes larger, partial discharge occurs around the foreign substance and supplies electric charges to the foreign substance.
When the partial discharge occurs, the foreign substance suddenly moves in a wide range to affect the insulation performance. Besides, when an overvoltage such as lightning surge enters and thereby the electric field on the inner surface of the metal container becomes large, the electric field at the electric field concentrated portion sometimes becomes much larger, so that the foreign substance suddenly greatly moves around.
To suppress the sudden movement of the foreign substance around in a wide range, it is necessary to relax the electric field concentration between the insulation resistant material and the foreign substance to thereby suppress occurrence of partial discharge and field emission. Besides, a higher voltage such as a lightning impulse causes partial discharge to cause the foreign substance to start to move.